1. Field of the Invention
The present invention relates to methods and apparatus for estimating the accuracy of measurement signals and, more particularly, to techniques for preventing use of spurious or low-quality range measurement signals in determining an object""s position and for estimating the accuracy of acceptable range measurement signals based on measurement history information and operational parameters.
2. Description of the Related Art
Tracking filters are commonly used in a variety of contexts to estimate the present state of an entity by processing raw measurements relating to the entity. For example, the relative or absolute position of an object can be determined by taking measurements, such as series of range measurements, that indicate position. Typically, such measurements have some degree of inaccuracy due to the presence of noise or interference which introduces errors in the measurement values. By tracking the position of the object over time, a tracking filter essentially reduces the uncertainty caused by measurement noise and develops a more accurate estimate of the object""s position than would be possible from simply assuming that each measurement accurately reflects the object""s true position.
Minimal means-square-error (MSE) filters, such as the well-known Kalman filter, attempt to minimize errors in the tracked position of an object by appropriately weighting the impact of each measurement as a function of the reported accuracy of the measurement. When a new measurement is received, the filter predicts the position of the object at the present time by extrapolating from the previously estimated state of the object. The filter also estimates the accuracy of the predicted current position. The accuracy of the measured position is conventionally determined as a function of the received signal-to-noise ratio (or the signal-to-interference ratio where significant interference is present in addition to noise), with a higher signal-to-noise ratio translating into a higher measurement accuracy. To update the state of the object (e.g., the estimated position and velocity in three dimensions) with the new measurement, the filter must decide the relative extent to which it trusts the predicted current position and the new measurement. If the accuracy of the measurement is high relative to the accuracy of the predicted position, the filter will incorporate the measurement into the position solution using a high filter gain, meaning that the updated position estimate will rely more heavily on the measurement than on the predicted position. Conversely, if the accuracy of the measurement is low relative to the accuracy of the predicted position, the filter will incorporate the measurement into the position solution using a low filter gain, meaning that the updated position estimate will rely more heavily on the predicted position than on the measurement, such that the measurement will have less impact on the position estimate generated by the tracking filter.
Although the signal-to-noise ratio is conventionally relied upon to gauge the accuracy of position measurements in the filter updating process, there are circumstances in which the signal-to-noise ratio alone may not fully reflect the accuracy of the measurement or the extent to which the tracking filter should rely on the measurement. In the case of measuring the range to an object or another device, a precise determination of the signal propagation time between the devices must be made. The signal propagation time can be derived by knowing the transmission and reception times of one or more ranging signals traveling along a direct path between the devices.
For example, the well-known global positioning system (GPS) relies on measurement of the one-way propagation time of signals sent from each of a set of satellites to a receiving device in order to determine the range to each satellite and the position of the receiving device. Position location systems that relies on a two-way, round-trip ranging signal scheme are described in U.S. patent application Ser. No. 09/365,702, filed Aug. 2, 1999, entitled xe2x80x9cMethod and Apparatus for Determining the Position of a Mobile Communication Device Using Low Accuracy Clocksxe2x80x9d and U.S. patent application Ser. No. 09/777,625 filed Feb. 6, 2001, entitled xe2x80x9cMethods and Apparatus for Determining the Position of a Mobile Communication Devicexe2x80x9d, the disclosures of which are incorporated herein by reference in their entireties. In the ranging schemes described in these applications, a master mobile communication device transmits outbound ranging signals to plural reference communication devices which respond by transmitting reply ranging signals that indicate the location of the reference radio and the signal turn around time (i.e., the time between reception of the outbound ranging signal and transmission of the reply ranging signal). Upon reception of the reply ranging pulse, the master radio determines the signal propagation time, and hence range, by subtracting the turn around time and internal processing delays from the elapsed time between transmission of the outbound ranging pulse and the time of arrival of the reply ranging pulse. The accuracy of the position determined by these systems depends largely on the accuracy with which the receiving devices can determine the time of arrival of the ranging signals traveling along a direct path between the devices.
In an environment where multipath interference is significant, it is possible to mistakenly identify a strong multipath signal as the direct path signal. Since a multipath signal travels along an indirect path between the transmitter and receiver, the signal propagation time and, hence, the observed range differ from that of the direct path. In a position determining system relying on precise measurements of direct-path signal propagation time to determine range, erroneously interpreting a multipath signal as the direct path signal can drastically degrade performance. In particular, a multipath signal may result in a severely erroneous range measurement; nevertheless, if the multipath signal has a relatively high signal-to-noise ratio, the erroneous range measurement will be reported to the tracking filter as being highly accurate. Consequently, the filter will be misled into placing a high degree of reliance on a severely erroneous range measurement, thereby degrading the accuracy of the position estimate without the degraded accuracy being immediately known or reported.
As described in the aforementioned patent applications, one approach to avoiding the problem of accuracy degradation caused by multipath signals is to use frequency diversity to find a transmission frequency and phase that minimize multipath interference. A rake filter or equalizer can also be employed to separately identify the direct path signal and prominent multipath signals in order to separate or constructively combine these signals. Nevertheless, even with technique such as these, it is possible to measure range with a significant error that is not correctly represented by the signal-to-noise ratio of the ranging signal from which the range measurement is derived.
Even where the signal-to-noise ratio can be trusted as a indicator of measurement accuracy, there may be other measurement information available to supplement the signal-to-noise ratio in estimating the measurement accuracy. For example, the receiving device may have knowledge of the severity of multipath interference and the precision with which the signal arrival time is determined, and the history of recent measurements may suggest the extent to which the latest measurement should be relied upon. Failure to account for such factors in reporting the accuracy of the measurement to a tracking filter may result in a less accurate estimate of position. Accordingly, there remains a need to identify and prevent the use of spurious or unacceptably low accuracy measurements in systems that perform position estimation from measurement signals as well as a need for a better approach to estimating the accuracy of measurement signals supplied to a tracking filter that determines position.
The improved accuracy in position determination that would result from elimination of erroneous measurements would be of great benefit in a variety of applications. In a military context, it is desirable to know the location of military personnel and/or equipment during coordination of field operations and rescue missions. More generally, appropriately equipped mobile communication devices could be used to more accurately track the position of personnel and resources located both indoors or outdoors, including but not limited to: police engaged in tactical operations; firefighters located near or within a burning building or forest fire; medical personnel and equipment in a medical facility or en route to an emergency scene, including doctors, nurses, paramedics and ambulances; and personnel involved in search and rescue operations. A more accurate position location system would enhance capabilities to track and locate high-value items, including such items as personal computers, laptop computers, portable electronic devices, luggage, briefcases, valuable inventory, and automobiles. In urban environments, where conventional position determining systems have more difficulty operating, it would be desirable to more reliably track fleets of commercial or industrial vehicles, including trucks, buses and rental vehicles. Tracking of people carrying a mobile communication device is also desirable in a number of contexts, including, but not limited to: children in a crowded environment such as a mall, amusement park or tourist attraction; location of personnel within a building; and location of prisoners in a detention facility. The capability to accurately determine the position of a mobile communication device also has application in locating the position of next-generation cellular telephones. The capability to determine the position of cellular telephones could be used to pinpoint the position from which an emergency call has been made. Such information could also be used to assist in cell network management (for example, by factoring each mobile communication device""s position into message routing algorithms).
Therefore, in light of the above, and for other reasons that become apparent when the invention is fully described, an object of the present invention is to improve the accuracy of the estimated state of a tracked entity, such as the estimated position of an object or a mobile communication device.
More particularly, it is object of the present invention to accurately determine the three-dimensional position of a mobile communication device in a variety of environments, including urban areas and inside buildings where multipath interference can be great.
Another object of the present invention is to identify and prevent the use of spurious or low-accuracy measurement signals in updating the state of an entity being tracked, such as the position of an object, even when the signal-to-noise ratio of the measurement signals erroneously suggests an acceptable degree of measurement accuracy.
A further object of the present invention is to minimize the effects of interference caused by multipath signal propagation in a position location system, thereby providing highly accurate three-dimensional position estimates even under severe multipath conditions.
Yet another object of the present invention is to account for a variety of measurement and operational information, in addition to or in place of signal-to-noise ratio, in order to report a more refined estimate the accuracy of measurement signals to a tracking filter.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with the present invention, range measurements useful for determining an object""s position are screened and evaluated so that only acceptable range measurement are supplied to a tracking filter and used to update the position solution. Range measurements deemed to be unacceptable are identified by the screening process and discarded before the erroneous range measurements can corrupt or degrade the position solution.
In an exemplary embodiment, the range measurement screening process is a two-stage screening technique involving a coarse screening stage and a fine screening stage. The coarse screening stage includes computing an estimated expected range between the reference radio which sent the measured ranging signal and the local receiving radio based on the positions of the reference and local radios estimated by their respective tracking (Kalman) filters. A coarse screening window centered about the estimated expected range is computed as a function of the estimated accuracies of the estimated positions of the reference and local radios.
If the range measurement falls outside the coarse screening window, the range measurement is declared too inaccurate for use in the tracking filter and is not used to update the position solution. If the range measurement falls within the coarse screening window, the range measurement is next evaluated using a fine screening process. The fine screening process relies on a comparison of the range measurement to a measurement history, specifically, a fading average of previous range measurements and the variability of these previous range measurements. An estimated range used to center the fine screening window is calculated from a fading average of the last N measurements used to update the position solution. The width of the fine screening window is a function of the standard deviation of the last N measurements used to calculate the estimated fading-average range.
If the range measurement falls outside the fine screening window, the range measurement is declared too inaccurate for use and is not used to update the position solution. If the range measurement falls within the fine screening window, an estimate of the range measurement accuracy is developed based on one or more of the following factors: the standard deviation of the last N range measurements; the difference (or xe2x80x9cerrorxe2x80x9d) between the range measurement and the estimated fading-average range; the number of rake taps used in the receiver; the signal-to-noise ratio of the range measurement; a xe2x80x9cquality of fitxe2x80x9d metric indicative of measurement timing accuracy; and the variance of the estimated positions of the local and reference radios. These parameters can be used to determine a more refined estimate of the range measurement accuracy than would be possible by relying on the signal-to-noise ratio alone.
For each acceptable range measurement, the estimated range measurement accuracy is supplied along with the range measurement itself to the navigation system Kalman filter. The Kalman filter uses the estimated range measurement accuracy to appropriately weight the impact of the range measurement in updating the position solution. The criteria for acceptability and accuracy is dynamically adjustable so that the screening and accuracy estimation process can be optimized for various operating environments. For example, the measurement acceptability thresholds (i.e., the size of the screening windows) can be dynamically adjusted based upon factors such as the number of measurements available and the variability of the measurements in a particular environment.
The measurement screening and accuracy estimation techniques of the present invention can be used to enhance the accuracy of position determination systems useful in wide variety of applications, including location and/or tracking of people and items such as: military personnel and equipment, emergency personnel and equipment, valuable items, vehicles, mobile telephones, children, prisoners and parolees.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein.